Perfetto IPC

This doc is WIP, stay tuned.

TL;DR
We needed an IPC for Android and Linux which was small, simple, controllable, predictable, C++11 friendly and debuggable. Our IPC transport is not mandatory outside of Android, you can wrap your own IPC transport (e.g., Perfetto uses Mojo in chromium) or just short circuit the Perfetto {Service,Producer,Consumer} interfaces for IPC-less full in-process use.

Key features:

• Protobuf over a unix-socket.
• Allows to send file descriptors over the wire: for setting up shared memory and passing the FD for the output trace from a consumer to the service.
• Service definition uses same protobuf rpc syntax of gRPC
• Extremely simple wire protocol.
• C++11 friendly, allows to bind std::function to each request.
• Leak (un)friendly: tries hard to guarantee that callbacks are left unresolved, using C++11 move semantics.
• Shutdown / destruction friendly: tries hard to guarantee that no callbacks are issued after the IPC channel is destroyed.
• Disconnection-friendly: all outstanding requests (and hence pending callbacks) are nack-ed in case of a disconnection (e.g., if the endpoint crashes).
• Memory friendly: one virtually contiguous cache-friendly rx buffer, madvise()'d when when not used.
• Debugging friendly: single-thread only, based on non-blocking socket I/O.
• Binary size friendly: generates one protobuf per message, doesn't have any external dependency.
• Hopefully safe:
  • The rx buffer has guard regions around.
  • The wire protocol is based on protobuf.
  • Fuzzed
• Offers direct control of socket buffers and overrun/stalling policy.
• ABI stable.

Realistically will never support:

• Multithreading / thread pools.
• Congestion or flow control.
• Non-data object brokering (e.g. sending a remote interface).
• Introspection / reflection.